Switching devices are used in electronic applications to connect and disconnect electrical signal paths. One type of switching device that is useful in Radio Frequency (RF) and microwave applications is a Micro-electromechanical (MEMS) switch because of its low insertion loss and high isolation capability compared to transistor switches. In U.S. Pat. No. 5,619,061, Goldsmith et al. disclose a MEMS switch having a double hinge membrane-type control electrode with center flex. The membrane-type electrode is generally rectangular in shape having two ends. Each end of the electrode is hinged or anchored to a post, spacer, via or other type of stationary vertical structure. Thus, when the appropriate voltage is applied between the two control electrodes, the membrane-type hinged electrode flexes at the center, i.e., between the two hinges, in the direction of the opposite electrode, thereby closing the circuit. When the voltage is removed, the natural resiliency of the membrane-type electrode returns it to its normally horizontal, open state.
A double hinge structure for a digital micro-mirror device (DMD) is disclosed in U.S. Pat. No. 5,652,671. Rather than an electrical switch, a DMD is a light switch in which a micro-mirror either reflects light to, or deflects light from, an image plane, depending on the orientation of the mirror. This patent illustrates a double hinge torsion flex structure formed by the support posts, alumina layer, aluminum layer and mirror. A torsion moment, i.e. a twisting of the hinge, is used to orient the mirror for either reflection or deflection, i.e., partial rotation about the hinges, rather than a downward flexing moment. This configuration is similar to the double hinge cantilever approach discussed hereinbefore, which also depends on a torsion moment.
The drawbacks of the double hinge approach manifest themselves in at least two ways, i.e. thermal stress and unpredictable bi-stable states. The double hinge electrode is typically comprised of a metal layer, which carries a voltage potential, and a dielectric layer for restricting electric current and providing support to the flexible membrane. As the two layers, each with a unique thermal coefficient, expand and contract over temperature variation during operation, thermal stresses within each of the layers are created. Mechanical failure due to thermal stress typically occurs at the hinge because this is where the electrode is anchored and the stresses are concentrated. Thus, thermal expansion and contraction is restricted by the hinge.
Also, because the thermal expansion coefficients between the metal layer and the dielectric layer are dissimilar, the two layers expand and contract at different rates which results in a large thermal stress at the interface between the layers. Again, the most vulnerable point along the interface is at the hinges, where movement is restricted from the expansion and contraction of opposing forces.
Another failure mechanism common to double hinge micro electromechanical switches is an unpredictable bi-stable state formed by excess compressive stress after removal of a sacrificial layer between the upper and lower electrodes. During fabrication, a sacrificial or spacer layer is situated between the two control electrodes to provide the proper distance or spacing between the electrodes. Upon removal of the sacrificial layer, the membrane-type upper electrode may experience an unpredictable bowing phenomenon, primarily as a result of the differential stresses. If bowing in the downward direction occurs, the switch is permanently closed and is therefore unusable. If bowing in the upward direction occurs, one of two results is possible: either the switch is permanently open because an excessive actuation voltage is required or the switch will permanently close on the first application of the actuation voltage. In either case, the switch is unusable.
Stiction is another problem occurring in micro electromechanical switches. Stiction is defined, for the purposes of the present invention, as an adhesive and/or electro-static attraction between electrodes. Stiction has a negative impact on the switching speed and response of the device. When the voltage potential is removed, the electrodes should separate instantaneously. Any residual, unwanted attraction between electrodes will increase the time for separation, thereby decreasing switching speed. In extreme cases, stiction may bind the MEMS switch in the closed position, thus rendering it inoperable.
Therefore, a need exists for a more robust MEMS switch with increased reliability, reduced stiction, and higher switching speeds.